
where \(\zeta =(\zeta _1,\zeta _2,\zeta _3)\)  is the multi-index
and \({\nabla }^\zeta := \partial _{x_1}^{\zeta _1} \partial _{x_2}^{\zeta _2} \partial _{x_3}^{\zeta _3}\) .
For any \(A\subseteq , set{\begin{@align*}{1}{-1}N_{\varepsilon _*}(A):=\lbrace t\in \ \exists s\in A,\ s.t.\ |t-s|\le \varepsilon _*\rbrace .\end{@align*}}\) We also adapt the notations from [1]}.
Let \(u,\,v\)  be two vector fields,
the corresponding second order tensor product is defined by
\(u\otimes v:=(u_iv_j)_{1\le i,j\le 3}.\) 
